The mast cell is an attractive model system for studies of cellular differentiation because the phenotypic changes from progenitor cell to mast cell are great and the signals required to induce differentiation appear to be few as compared with other cells such as B lymphocytes. We have defined a late, committed stage in the differentiation of the mast cell progenitor just prior to granulation. This mast cell-committed progenitor does not require IL-3 for differentiation but does require a factor or factors provided by a connective tissue microenvironment such as murine embryonic skin. If the mast cell-committed progenitor can be successfully isolated and the connective tissue factors identified, a well-defined in vitro differentiation system could be developed to individually examine discrete changes that account for the mast cell phenotype. First, we propose to develop a new in vitro differentiation system whereby any of several mast cell features can be studied by following the response of late-stage progenitor cells to defined factors. For the goal to be realized, the following strategy will be used: We will obtain a purified preparation of mast cell- committed progenitors based on affinity for embryonic skin monolayers. The phenotype of these progenitors will be determined. We will precisely identify the factor or factors in monolayers of murine embryonic skin which are required for differentiation of the mast cell committed progenitor into mast cells. When this is done, the isolated progenitor cells will be triggered with purified preparations of the required factor to examine changes in activation events, expression of high-affinity IgE receptors and growth factor receptors, and the ability to dedifferentiate. Second, we propose to use the mast cell-committed progenitor culture system described in the grant proposal to determine if there are any known IgE regulatory factors which also affect mast cell differentiation. Third, we propose to adapt this differentiation system for rat studies in order to detect chymase, carboxypeptidase A, or heparin proteoglycan in mast cells derived from culture on embryonic skin monolayers. This will allow us to study mast cell heterogeneity (mucosal vs connective tissue type in an in vitro system.